Laser desorption/ionization mass spectrometry (hereinafter sometimes referred to as LDI-MS) is a method for carrying out mass spectrometric analysis of a sample by applying a laser to a sample to cause vaporization and ionization of sample molecules, detecting the ionized sample molecules by a detector and measuring the mass-to-charge ratio (m/z).
As LDI-MS, matrix-assisted laser desorption/ionization mass spectrometry (hereinafter sometimes referred to as MALDI-MS) has been known in which a sample is mixed with a matrix (for example, a low molecular weight organic compound) as an ionization-assisting substance and a cationizing agent, the mixture is irradiated with a laser, and sample molecules are vaporized by a heat energy generated by the matrix absorbing the laser while decomposition of the sample molecules is suppressed, and at the same time, the sample molecules are cationized by cations derived from the cationizing agent.
However, in the case of MALDI-MS, in a mass spectrum indicating the signal intensity by the y-axis and m/z by the x-axis, not only peaks derived from the sample but also peak derived from the matrix are observed, and the peaks derived from the sample can hardly be distinguished.
As LDI-MS not employing a matrix, surface-assisted laser desorption/ionization mass spectrometry (hereinafter sometimes referred to as SALDI-MS) has been developed in which a sample is placed on a sample plate having a surface which can exhibit an ionization-assisting effect, and by a thermal energy generated by the sample plate absorbing the laser, sample molecules are vaporized and cationized.
As a sample plate used in SALDI-MS, for example, the following have been proposed.
(1) A porous silicon plate having pores with a size of several hundreds nm, formed by electrolytic etching (Non-Patent Document 1).
(2) A sample plate formed by dropping a liquid containing Pt nanoparticles or Au nanoparticles on the surface of a commercially available LDI plate formed of e.g. stainless steel, followed by drying so that the nanoparticles are supported (Non-Patent Document 2, Patent Document 7).
(3) A sample plate having e.g. a metal thin film formed on the surface of a substrate having microstructures such as porous structures on its surface (Patent Documents 1 to 4).
(4) A sample plate having metal nanostructures (such as metal nanorods or dendritic platinum nanostructures) formed on the surface of a substrate (Patent Documents 5 and 6).
(5) A sample plate formed by dropping a liquid containing an oxide powder of Fe or Co on the surface of a substrate, followed by drying, so that nanoparticles are supported (Patent Document 8).